1. Field of the Invention
The present invention relates to a nitride semiconductor laser diode including a semiconductor multilayer structure made of group III nitride semiconductors.
2. Description of Related Art
Group III-V semiconductors employing nitrogen as a group V element are called “group III nitride semiconductors”, and typical examples thereof include aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN), which can be generally expressed as AlXInYGa1-X-YN (0≦X≦1, 0≦Y≦1 and 0≦X+Y≦1).
Short wavelength laser sources such as blue and green laser sources are increasingly employed in the fields of high-density recording in optical disks represented by a DVD, image processing, medical instruments, measuring instruments and the like. Such a short wavelength laser source is constituted of a laser diode employing GaN semiconductors, for example.
A GaN semiconductor laser diode is manufactured by growing group III nitride semiconductors on a gallium nitride (GaN) substrate having a major surface defined by a c-plane by metal-organic vapor phase epitaxy (MOVPE). More specifically, an n-type GaN contact layer, an n-type AlGaN cladding layer, an n-type GaN guide layer, an active layer (light emitting layer), a p-type GaN guide layer, a p-type AlGaN cladding layer and a p-type GaN contact layer are successively grown on the GaN substrate by the metal-organic vapor phase epitaxy, to form a semiconductor multilayer structure consisting of these semiconductor layers. The active layer emits light by recombination of electrons and positive holes injected from the n-type layer and the p-type layer respectively. This light is confined between the n-type AlGaN cladding layer and the p-type AlGaN cladding layer, and is propagated in a direction perpendicular to a stacking direction of the semiconductor multilayer structure. Cavity end faces are formed on both ends in the propagation direction, so that the light is resonantly amplified while repeating induced emission between the pair of cavity end faces and part thereof is emitted from either cavity end face as the laser beam.
One of important characteristics of the semiconductor laser diode is a threshold current (oscillation threshold) for causing laser oscillation. The energy efficiency of laser oscillation is improved as the threshold current is reduced.
However, the light emitted from the light emitting layer grown on the major surface defined by the c-plane is randomly polarized, and hence the ratio of the light contributing oscillation of a TE mode is small. Therefore, the efficiency of laser oscillation is not necessarily excellent but must be improved for reducing the threshold current.